Leukotrienes (LTs) and the biosynthetic pathway from arachidonic acid leading to their production have been the targets of drug discovery efforts for over twenty years. LTs are produced by several cell types including neutrophils, mast cells, eosinophils, basophils monocytes and macrophages. The first committed step in the intracellular synthesis of LTs involves oxidation of arachidonic acid by 5-lipoxygenase (5-LO) to LTA4, a process requiring the presence of the 18 kD integral membrane protein 5-lipoxygenase-activating protein (FLAP) (D. K. Miller et al., Nature, 1990, 343, 278-281; R. A. F. Dixon et al., Nature, 1990, 343, 282-284). Subsequent metabolism of LTA4 leads to LTB4, and the cysteinyl LTs-LTC4, LTD4 and LTE4 (B. Samuelsson, Science, 1983, 220, 568-575). The cysteinyl LTs have potent smooth muscle constricting and bronchoconstricting effects and they stimulate mucous secretion and vascular leakage. LTB4 is a potent chemotactic agent for leukocytes, and stimulates adhesion, aggregation and enzyme release.
Much of the early drug discovery effort in the LT area was directed towards the treatment of allergy, asthma and other inflammatory conditions. Research efforts have been directed towards numerous targets in the pathway including antagonists of LTB4 and the cysteinyl leukotrienes LTC4, LTD4 and LTE4, as well as inhibitors of 5-lipoxygenase (5-LO), LTA4 hydrolase and inhibitors of 5-lipoxygenase activating protein (FLAP) (R. W. Friesen and D. Riendeau, Leukotriene Biosynthesis Inhibitors, Ann. Rep. Med. Chem., 2005, 40, 199-214). Years of effort in the above areas have yielded a few marketed products for the treatment of asthma including a 5-LO inhibitor, zileuton, and LT antagonists, montelukast, pranlukast and zafirlukast.
More recent work has implicated LTs in cardiovascular disease, including myocardial infarction, stroke and atherosclerosis (G. Riccioni et al., J. Leukoc. Biol., 2008, 1374-1378). FLAP and 5-LO were among the components of the 5-LO and LT cascade found in atherosclerotic lesions, suggesting their involvement in atherogenesis (R. Spanbroek et al., Proc. Natl. Acad. Sci. U.S.A., 2003, 100, 1238-1243). Pharmacological inhibition of FLAP has been reported to decrease atherosclerotic lesion size in animal models. In one study, oral dosing of the FLAP inhibitor MK-886 to apoE/LDL-R double knockout mice fed a high-fat diet from 2 months of age to 6 months led to a 56% decrease in plaque coverage in the aorta and a 43% decrease in the aortic root (J. Jawien et al., Eur. J. Clin. Invest., 2006, 36, 141-146). This plaque effect was coupled with a decrease in plaque-macrophage content and a concomitant increase in collagen and smooth muscle content which suggests a conversion to a more stable plaque phenotype. In another study, it was reported that administration of MK-886 via infusion to ApoExCD4dnT RII mice (apoE KO mice expressing a dominant-negative TGF-beta receptor which effectively removes all TGF-beta from the system) resulted in about a 40% decrease in plaque area in the aortic root (M. Back et al., Circ. Res., 2007, 100, 946-949). The mice were only treated for four weeks after plaque growth was already somewhat mature (12 weeks) thus raising the possibility of therapeutically treating atherosclerosis via this mechanism. In a study examining human atherosclerotic lesions, it was found that the expression of FLAP, 5-LO and LTA4 hydrolase was significantly increased compared to healthy controls (H. Qiu et al., Proc. Natl. Acad. Sci. U.S.A., 103, 21, 8161-8166). Similar studies suggest that inhibition of the LT pathway, for example by inhibition of FLAP, would be useful for the treatment of atherosclerosis (for reviews, see M. Back Curr. Athero. Reports, 2008 10, 244-251 and Curr. Pharm. Des., 2009, 15, 3116-3132).
In addition to the work cited above, many other studies have been directed towards understanding the biological actions of LTs and the role of LTs in disease. These studies have implicated LTs as having a possible role in numerous diseases or conditions (for a review, see M. Peters-Golden and W.R. Henderson, Jr., M.D., N. Engl. J. Med., 2007, 357, 1841-1854). In addition to the specific diseases cited above, LTs have been implicated as having a possible role in numerous allergic, pulmonary, fibrotic, inflammatory and cardiovascular diseases, as well as cancer. Inhibition of FLAP is also reported to be useful for treating renal diseases such as diabetes-induced proteinuria (see for example J. M. Valdivieso et al., Journal of Nephrology, 2003, 16, 85-94 and A Montero et al., Journal of Nephrology, 2003, 16, 682-690).
A number of FLAP inhibitors have been reported in the scientific literature (see for example J. F. Evans et al., Trends in Pharmacological Sciences, 2008, 72-78) and in U.S. patents. Some have been evaluated in clinical trials for asthma, including MK-886, MK-591, and BAY X1005, also known as DG-031. More recently, the FLAP inhibitor AM-103 (J. H. Hutchinson et al., J. Med. Chem. 52, 5803-5815) has been evaluated in clinical trials, based on its anti-inflammatory properties (D. S. Lorrain et al., J. Pharm. Exp. Ther., 2009, DOI:10.1124/jpet.109.158089). Subsequently, it was replaced by the back-up compound AM-803 (GSK-2190915) for the treatment of respiratory diseases. DG-031 has also been in clinical trials to evaluate its effect on biomarkers for myocardial infarction risk and showed a dose-dependent suppression of several biomarkers for the disease (H. Hakonarson et al., JAMA, 2005, 293, 2245-2256). MK-591 was shown in a clinical trial to reduce proteinuria in human glomerulonephritis (see for example A. Guash et al., Kidney International, 1999, 56, 291-267).
However, to date, no FLAP inhibitor has been approved as a marketed drug.